The invention relates to a method of manufacturing glass bodies, in which a porous green body is formed from the starting material for the glass body, which green body is then purified and sintered, the starting material being an aqueous suspension comprising a highly disperse solids content, and deposited by separating the phased of the suspension through electrophoresis, in such a manner that the solids content of the suspension is deposited on a porous deposition membrane having pores whose diameter is smaller than the average particle diameter of the solid particles present in the suspension, said membrane being arranged between the anode and the cathode, and the space between said membrane and the anode being filled with an electrically conducting liquid.
The invention furher relates to devices for carrying out the method in accordance with the invention.
The method described in the opening paragraph is particularly suitable for the manufacture of highly pure glass bodies. Highly pure silica-glass tubes are used, for example, as substrate tubes for the manufacture of optical waveguides by means of an internal coating process, in which tubes impurities in the form of subgroup elements and OH-groups are permissible to a degree of only a few ppb. For example, structured preforms for optical waveguides can be manufactured by combining very pure silica-glass rods with fluorine-doped cladding glasses.
Discs of highly pure silica glass are used as, for example, mask carriers in the semiconductor technology, a transmission of radiation in the ultraviolet range of the spectrum (180 nm) being required. For this reason, also this application of silica glass permits only a very low level of impurities, in particular in the form of metal ions.
German Patent Application DE 35 11 451, which corresponds to U.S. Pat. No. 4,684,386 describes a method of and a device for manufacturing glass bodies, which method and device enable a porous green body to be formed from the starting material for the glass body, being an aqueous suspension comprising a highly disperse solids content, which green body is subsequently purified and sintered, and deposited by separating the phases of the suspension through electrophoresis.
In the case of electrophoretic deposition from aqueous suspensions, local changes of the pH-value in the suspension take place as a result of the discharge of H.sup.+ -or OH-ions at the electrodes, which may lead to a change of the colloid-chemical system. An effect on the electrophoretic deposition can be observed, in particular, when due to long deposition times caused by low deposition rates or a large output of solid matter there is sufficient time to thoroughly mix the suspension in the deposition chamber, or when the electrolytes added to the suspension have only a small buffer effect due to their degree of dissociation or the concentration.
Changes in the effective pH-value during the electrophoretic deposition influence the density of the deposited moulded body, as a result of which geometrical changes or cracks may develop in subsequent processing steps (drying, sintering).
Further, it has been found that the density of the suspension decreases according as the rate of deposition of the dispersed particles increases. In the manufacture of very long tubes or in the case of perpendicularly arranged devices having a relatively large ratio of deposited material to dispersed particles, this may lead to separation problems. The concentrated suspension sediments and is covered by the depleted suspension. Since the electrophoretic deposition rate depends on the concentration of the particles in the suspension, the concentration gradient of the suspension may bring about a decrease in layer thickness from the bottom upwards.